1. Field of the Invention
This invention relates to a base station transmission control method, a cellular system and a base station. It relates in particular to a base station transmission control method, and to a cellular system and a base station, for controlling the transmission of base station control signals during handover in a cellular system.
2. Description of Related Art
A cellular system employing direct sequence code division multiple access (DS-CDMA) uses the same frequency band in a plurality of channels. As a result, from the point of view of a given channel, radio waves in other channels constitute interference, and an increase in the power of this interference can result in a deterioration of reception quality for the desired signal, and in the connection being dropped, etc. Hence the number of channels through which communication can be sustained at the required reception quality, in other words, the channel capacity, is dependent on the amount of interference. In the uplink, a signal transmitted by a mobile station located far from a base station undergoes more attenuation than a signal transmitted by a mobile station located near that base station, and hence transmitting such signals at the same power gives rise to the near-far effect in which the interfering signal becomes stronger than the desired signal and communication becomes difficult. Transmission power control is therefore an essential technique for uplink channels, where it involves controlling the transmission power of the mobile stations so that their signals are received at a base station at the same received power.
Although a near-far effect of this sort does not occur for downlink channels, in order to reduce interference to other channels, transmission power control is performed at base stations so that transmission power is kept at the minimum necessary for mobile stations to maintain the required reception quality. Control of transmission power at the base stations is based on closed-loop control. This involves comparing the measured reception quality with a prescribed target quality, and if it is found to be higher than the target quality, a transmission power control (TPC) signal instructing that transmission power be decreased is transmitted, while if the measured reception quality is found to be lower than the target quality, a TPC signal instructing that transmission power be increased is transmitted.
Customarily, a technique called soft handover is used in a cellular system employing CDMA. Soft handover involves transmitting from a plurality of base stations to which a mobile station is connected simultaneously, and is employed when a mobile station approaches the vicinity of a cell boundary and the difference between the propagation loss experienced by the signal received from the base station to which the mobile station is connected and the propagation loss experienced by the signal received from a neighbouring base station falls within a prescribed threshold. Due to the diversity effect obtained by transmitting from a plurality of base stations, soft handover results in improved reception quality when a mobile station is near a cell boundary, where propagation loss is large and reception quality tends to deteriorate. Because soft handover sets up a connection to a base station which is the candidate for the next connection before the connection to the active base station is released, handover is smooth and without any momentary loss of signal. Because a mobile station is generally at a different distance from each of several base stations, each of these base stations sees a different uplink reception quality during soft handover. Hence in the IMT-2000 standard, for example, during soft handover each base station notifies the base station controller of the reception quality of the signal obtained by RAKE combining, whereupon selection combining is performed using the signal with the highest reception quality as the desired signal. Uplink transmission power control in the IMT-2000 standard therefore involves comparing the uplink reception quality at each base station with a prescribed target reception quality, forming TPC signals on the basis of these results, and transmitting these TPC signals to the mobile station. If even one of the plurality of TPC signals received at the mobile station instructs a decrease in transmission power, the mobile station decreases its transmission power, while if all the TPC signals instruct an increase in transmission power, the mobile station increases its transmission power. This provides transmission power control that reduces any excess uplink transmission power while satisfying the required reception quality.
On the other hand, a problem encountered in the downlink is that because a plurality of base stations are transmitting during soft handover, there is an increase in interference and hence a decrease in channel capacity. As a solution to this problem, JP H11-069416 A discloses a technique for restricting the base stations that transmit during soft handover, which serves to decrease the downlink interference. Specifically, base stations transmit a pilot signal at a prescribed power, and the mobile station measures the reception quality of the pilot signals transmitted by those base stations that have set up a connection. The base station with the highest reception quality is taken as the transmitting base station, which is called as “primary cell”, while the other base stations stop transmitting. This ensures that only base stations with low propagation loss transmit, with the result that downlink interference is decreased and channel capacity can be expected to increase.
The site selection diversity TPC (SSDT) scheme in the IMT-2000 standard applies this technique. A base station usually transmits both dedicated physical control channel signals and dedicated physical data channel signals in the downlink. In SSDT, base stations other than the transmitting base station (i.e., non-transmitting base stations) halt the transmission of dedicated physical data channel signals only, and continue to transmit dedicated physical control channel signals (3G TS 25.214 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 1999) 5.2.1.4.5). The reason for this is that, as mentioned previously, each base station that has set up a connection to the mobile station has its own TPC signal and hence even the non-transmitting base stations have to use the dedicated physical control channel to transmit these TPC signals.
However, the proportion of dedicated physical control channel signals and dedicated physical data channel signals transmitted in the downlink varies according to transmission rate and other factors. For example, at low transmission rates, the proportion of dedicated physical control channel signals becomes higher than that of dedicated physical data channel signals. An excessively high proportion of dedicated physical control channel signals results in the problem that the dedicated physical control channel signals transmitted by non-transmitting base stations will have a significant interference impact on other channels, resulting in decreased channel capacity.
It may be pointed out that not transmitting any downlink dedicated physical control channel signals at all from the non-transmitting base stations would be tantamount to not transmitting any TPC signals for controlling transmission power in the uplink, which would give rise to the problem that uplink reception quality might deteriorate.